1. Field of the Invention
The present invention relates to an EUV (extreme ultra violet) light source device for generating extreme ultra violet light by irradiating a target with a laser beam.
2. Description of a Related Art
As semiconductor processes become finer, photolithography has been making rapid progress toward finer fabrication, and, in the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of 50 nm or less, the development of exposure equipment with a combination of an EUV light source of about 13 nm in wavelength and a reduced projection reflective optics is expected.
As the EUV light source, there are three kinds of light sources which include an LPP (laser produced plasma) type using plasma generated by irradiating a target with a laser beam, a DPP (discharge produced plasma) type using plasma generated by discharge, and an SR (synchrotron radiation) type using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity near black body radiation can be obtained because plasma density can be made considerably high, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π sterad can be ensured because the light source is a point light source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Accordingly, the LPP type EUV light source device is thought to be predominant as a light source for EUV lithography, which requires a power of several tens of watts.
FIG. 31 is a schematic diagram showing the structure of a general LPP type EUV light source device. The EUV light source device includes a plasma generation chamber 2 with a nozzle 1, a laser light source 3, an optical propagation system for guiding a laser beam to the plasma generation chamber 2 (e.g. a lens 4), and a vacuum pump 5. Hereinafter, the lens 4 is used as an example of the optical propagation system in the explanation.
The nozzle 1 forms a target jet passing through a laser irradiation point by injecting a liquid or gas target material with pressure. In the case where a material such as xenon (Xe), which is in a gas state at a room temperature, is used as the target material, there may be provided upstream of the nozzle a mechanism for turning the target material into a liquid state by cooling the target material with pressure. On the other hand, in the case where a material such as stannum (Sn) or lithium (Li) which is in a solid state at room temperature is used as the target material, there may be provided upstream of the nozzle a mechanism for turning the target material into a liquid state by heating the target material beyond the melting temperature.
Further, by providing piezoelectric element 6 to the nozzle 1 to inject the target material in a liquid state while vibrating the nozzle 1, liquid drops of the target material, that is, droplets 8 can be generated. According to Rayleigh's theory of stability in minute disturbance, when disturbing a target jet having a diameter “d” flowing at a velocity “v” by adding vibration having a frequency “f”, in the case where a wavelength λ(λ=v/f) of the vibration generated in the target jet meets a predetermined condition (for example, λ/d=4.51), droplets 8 having a uniform size are repetitively generated at the frequency “f”. The frequency at that time is called Rayleigh's frequency.
The laser light source 3 outputs a laser beam at a predetermined repetitive operation frequency. A laser beam output from the laser light source 3 is collected to a laser irradiation point 7 through a lens 4 and irradiates the target jet or droplets. Thereby, the target material is turned into a plasma state to emit the EUV light. In FIG. 31, there are shown marks of droplets 8a, which have been irradiated with a laser beam and turned into a plasma state. The EUV light thus generated is collected by the collector mirror having a curved surface on which Mo/Si films are formed for reflecting a light beam having a wavelength of 13 nm to 14 nm at a high reflection rate in the case of exposing a semiconductor device, and the collected EUV light is guided into an exposure device by means of a reflection mirror optical system. Since the EUV light is largely absorbed by a material or largely interacted with a material, a reflection type system is used as an optical system for guiding the EUV light to the exposure device and an optical projection system inside the exposure device.
The vacuum pump 5 exhausts inside of the plasma generation chamber 2 to keep the desired pressure and eject unwanted material such as an evaporation gas 9 of the target material. In order to prevent the gasified target material from absorbing the EUV light and preventing contamination of the optical system such as a mirror, a degree of vacuum of about 0.1 Pa is required when xenon is used as the target material.
Generally, a frequency “f” of the vibration is added to nozzle 1 to form a uniform size of droplets can be several times to several tens of times the repetitive operation frequency of outputting an irradiation laser beam. For example, the repetitive operation frequency of a YAG laser generally used in the LPP type EUV light source is about 10 kHz, while a frequency “f” for generating droplets by vibration is about 110 kHz in the case of forming droplets having a diameter of about 60 μm dropping at a velocity of about 30 m/s. Therefore, most of the generated droplets pass through the laser irradiation point 7 without being irradiated with a laser beam. Such droplets (remaining target material) 10 are exhausted to the outside of the plasma generation chamber 2 by the vacuum pump 5. However, in the case where only the vacuum pump 5 is provided to the plasma generation chamber 2, it is difficult to keep inside of the plasma generation chamber 2 at a high degree of vacuum. As a result, the generated EUV light is apt to be absorbed to the target material gasified in the plasma generation chamber 2, which reduces output of the EUV light. Especially, the EUV light having a wavelength of 13.5 nm to be used for EUV photolithography is easily absorbed by xenon gas, and therefore, the EUV generation efficiency is decreased.
As related art, Japanese Patent Application Publication JP-P2004-31342A discloses a laser plasma EUV radiation source preventing succeeding target droplets from being affected by ionized preceding droplets. A source nozzle of the EUV radiation source has an orifice with a predetermined dimension capable of ejecting droplets at a rate set by a natural Rayleigh unstable destructive frequency. The target material generated by a piezoelectric transducer. A droplet generation rate is decided by factors in relation to a pulse frequency from an exciting laser such that buffer droplets are applied between target droplets. The buffer droplets act for absorbing radiation from the ionized target droplet so as not to affect the succeeding target droplets. However, even if the buffer droplets absorb radiation from the ionized target droplet, a degree of vacuum or cleanness in the plasma generation chamber is decreased due to the buffer droplets, which causes reduced output of the EUV light.
Further, Japanese Patent Application Publication JP-P2003-518731A (WO01/049087) discloses providing a vacuum space (collecting chamber) for collecting unwanted target material droplets to an EUV light source device as shown in FIG. 31. That is, as shown in FIG. 32, the collecting chamber 11 provided with vacuum pump 12 is provided downstream of the plasma generation chamber 2. The collecting chamber 11 is connected to a plasma generation chamber 2 through a constriction part (e.g. skimmer or orifice) 13 having a opening portion 13a. Thereby, from among the droplets injected from the nozzle 1, droplets 10, which have not been irradiated with a laser beam and have not contribute to generation of plasma, are collected in the collecting chamber 11 through the opening portion 13a, and ejected outside by the vacuum pump 12. By properly determining a diameter of the opening portion 13a, an exhausting amount of the vacuum pump 12 and so on, a high degree of vacuum and cleanness in the plasma generation chamber 2 can be kept more easily than the EUV light source device as shown in FIG. 31. Further, by providing two vacuum pumps in total, a burden of each vacuum pump is reduced and a size of each vacuum pump can be reduced.
However, also in the EUV light source device as shown in FIG. 32, the target material always evaporates from a surface of the droplets 8 since droplets 8 are injected from the nozzle 1 until droplets 8 pass through the constriction part 13. Further, energy of an irradiation laser beam is very high. Therefore, several droplets preceding or succeeding the droplet to be irradiated with the laser beam are influenced by deformation or position change due to the impact of an irradiation laser, and at worst, evaporated by heat. Accordingly, not all of the droplets which have not been irradiated with a laser beam are collected to the collecting chamber 11. Such an evaporation gas or droplets not collected should be exhausted by the vacuum pump 5 provided in the plasma generation chamber 2. For that reason, it is difficult to lower a degree of vacuum to a required level (e.g. about 0.1 Pa in the case of employing xenon as a target material) in the plasma generation chamber 2. As a result, also in the EUV light source device, a degree of vacuum in the plasma generation chamber 2 cannot be kept at a high degree of vacuum, and the generated EUV light is absorbed, which causes reduced output of the EUV light.
In this regard, JP-P2003-518731A also discloses that a second vacuum space (collecting chamber) provided to the EUV light source device shown in FIG. 32. As shown in FIG. 33, the second collecting chamber 14 provided with the vacuum pump 15 is arranged at upstream of the plasma generation chamber 2. The collecting chamber 14 and the plasma generation chamber 2 are connected to each other through an opening portion 16a formed in a constriction part 16. Thereby, evaporation gas generated from droplets 8 injected from the nozzle 1 is exhausted to the outside by the vacuum pump 15, and therefore, a degree of vacuum or cleanness in the plasma generation chamber 2 can be kept higher than that of the EUV light source device as shown in FIG. 32.
However, in the plasma generation chamber 2, there are still droplets 10 not irradiated with a laser beam remaining. Such droplets are affected by adjacent droplets to be deformed or changed in a position due to impact of laser irradiation to the adjacent droplets, and at worst, evaporated by heat.
Thus, unexpected evaporation or the like occurs in the plasma generation chamber 2, and therefore, not all of the unwanted droplets are collected into the collecting chamber 11. Therefore, it is also difficult to keep the pressure at a high degree of vacuum in the plasma generation chamber 2 or to protect components such as a mirror in the chamber from the gasified target material. Further, three chambers and three vacuum pumps become required, which makes the system large and complicated.
The above-mentioned influence of the target turned into plasma affecting the adjacent targets becomes problem not only in the case where the target material is in a gas state at a room temperature, for example, xenon, but also in the case where the target material is in a solid state or a liquid state including a solid at a room temperature, for example, molten metal of stannum or lithium, a mixture in which minute metal particles of stannum, stannum oxide, copper or the like are dispersed into water or alcohol, or an ionic solution in which lithium fluoride or lithium chloride is dissolved into water. Those target materials may evaporate as a result of being affected by the heat of the plasma, and once gasified, metal particles contaminate components such as a mirror in the chamber, further degrading the performance of the EUV light source device.